(1) Field of the Invention
The present invention relates to a developing apparatus using a two-component type developer. More specifically, the present invention relates to a developing apparatus using a two-component type developer, in which stirring mixing of a toner and a carrier is effectively accomplished.
(2) Description of the Prior Art
As the method for developing an electrostatic latent image with a magnetic brush, there are known a developing method using a two-component type developer comprising a magnetic carrier and a toner and a developing method using a one-component type developer comprising toner particles alone. The former method is advantageous in that charges can be given to the toner stably, and the latter method is advantageous in that the troublesome operation of adjusting the toner concentration is not necessary and the property of reproducing a halftone is higher than in the former method.
Recently, it is required that a reproduced image should have a high contrast, that is, a clear shade, and a halftone should be beautifully reproduced. The developing method using a one-component type developer is effective for satisfying these requirements. However, since the upper threshold value of the developing condition is ordinarily determined by the force of magnetically attracting the developer to the developing sleeve and the Coulomb force to the electrostatic latent image and the magnetic material is dispersed in the binder resin in the one-component type developer, the magnetic attractive force of the one-component type developer to the sleeve is weaker than that of the magnetic carrier and hence, fogging is readily caused in the developing method using a one-component tyep developer. In order to solve this problem, in an ordinary developing method using a one-component type developer, the earing length of the magnetic brush is shortened so as to prevent occurrence of fogging. However, in this case, such troubles as insufficient earing and reduction of the flowability of the developer are caused and when the copying machine is continuously used, if a foreign substance such as paper dust is incorporated into the magnetic brush, stable earing becomes difficult.
Accordingly, research is continued with a view to improving the halftone-reproducing property by using a developing method using a two-component type developer where a stable magnetic brush can be formed and sufficient charges can be given to the toner. For example, there is known a method in which in the developing zone the moving direction of a photosensitive drum having an electrostatic latent image is made in agreement with the moving direction of the magnetic brush (hereinafter referred to as "the same direction development"). In this method, the development time can be prolonged and the sliding frictional force can be reduced, and therefore, this method is advantageous in the reproduction of a halftone over a method in which the moving direction of the photosensitive drum is reverse to the moving direction of the magnetic brush (hereinafter referred to as "the reverse direction development"). Moreover, the same direction development is advantageous in that leakage of the charge of the electrostatic latent image by the carrier can be prevented.
However, if this same direction development method is adopted, the size of the developing apparatus becomes larger than the size of the developing apparatus used for the reverse direction development method. The reason is as follows.
In case of the reverse direction development, in mixing and stirring a toner and a magnetic carrier (often referred to as "carrier" hereinafter), the developer in which the toner has been consumed in the developing zone is once scraped down from the surface of the developing sleeve by a scraper and the developer thus scraped down is sufficiently mixed and stirred with the newly supplied toner and the developer left in the developing apparatus. Then, the mixed and stirred developer is retained on the sleeve at a developer pumpup position located in the lower portion of the sleeve and is fed to the developing zone again after attainement of a certain earing length by an earing member. Namely, in the reverse direction development, by forming one loop of the above operations, mixing and stirring of the developer can be performed at a high efficiency. In contrast, in the same direction development, formation of an operation loop as mentioned above is difficult. Namely, in the case where a scraper is arranged in the lower portion of the sleeve to scrape down the used developer, it becomes necessary to form a reservoir zone below the position of the scraper so as to store the developer scraped down and a mechanism for pumping up the developer to the upper portion of the sleeve. If such a mechanism is not arranged and the scraper is stationarily arranged in the state buried in the developer, circulation of the developer in the developing apparatus is hindered and the significance of provision of the scraper is arranged in the upper portion of the sleeve as in case of the reverse direction development, because of the relation of the position of the scraper to the developer pump-up position, the loop becomes inevitably complicated. Accordingly, in each case the size of the developing apparatus is increased.